The statistics show a decrease in the proportion of GCSEs awarded at least a C grade in the core subjects of English, maths and science.

The Joint Council for Qualifications (JCQ), which publishes Thursday's national results, insisted that the drop in A*-C English results is partly down to more candidates sitting the exam earlier, during the winter exam season.

The number of entries for English GCSE, including English literature, has increased by 3.1%, JCQ said.

It added that there was a "dramatic" increase in entries for science GCSE - up 36.5% - and said that the fall in results at A*-C in this subject is partly due to a "more demanding standard" introduced this year, and a "significant" increase in entries by 15-year-olds.

Around 600,000 teenagers in England, Wales and Northern Ireland are receiving their GCSE results on Thursday.

Teachers have raised concerns that English exams have been marked too harshly, with schools reporting an unprecedented number of fails among their pupils.

The gap between girls and boys stalled at the very top grades, with 18.9% of boys' entries achieving an A* and A, compared to 25.6% of girls' entries - a percentage gap of 6.7%, the same as there was in 2011.

At grades A* to C, girls are pulling away, with 65.4% of boys' entries attaining that level, compared to 73.3% of girls' entries. Last year, 66% of boys' entries achieved A* to C, compared to 73.5% of girls' entries.

The long decline in the take-up of modern foreign languages appeared to be slowing this year, with even a rise of 10% in the number of those sitting Spanish GCSE.

The number of entries for French fell by 0.5%, compared to a 13.2% fall last year, and the entries for German fell by 5.5% compared to a 13.2% fall in 2011.

There was a rise of 13.7% in the uptake of other modern languages, including Arabic, Chinese, Persian, Polish, Portuguese and Italian, which all saw significant increases.

Michael Turner, director of the Joint Council for Qualifications, said: "These are a good set of results and students and teachers should be pleased with what they have achieved.

"It will be interesting to see if this year's rise in students taking Spanish and the rate of decline slowing in French and German is the beginning of a trend that will see more young people studying languages."

Andrew Hall, chief executive of exam board AQA, said: "This year has got more change in it than I think I've seen in my time at any awarding body."

AQA stressed that the standards students have to achieve remained the same.

Ziggy Liaquat, managing director of Edexcel, said: "The quality of work required to achieve an A grade this year is the same as the quality of work required to achieve an A grade last year."

The exam boards said changes were most apparent in the science results, with Mark Dawe, chief executive of the OCR, saying that the Government wanted a more difficult paper set for students.

Mr Hall said: "There is very, very clearly a new standard in science. We said it was coming, the Government said it wanted it and we have delivered it, and it shows in the outcomes."

This year's exam results shows the impact of the new English baccalaureate, with a reverse in the decline of numbers taking history and geography, as well as improvement in the take-up of modern foreign languages.

Mr Hall said: "I think it's quite encouraging. There is still a real crisis in modern foreign languages but maybe here we see the beginning of the EBacc starting to have an impact."

Exam board chiefs said this was a year of "major change" in English, maths and science.

As well as changes to GCSE science, new English courses have also been brought in.

Mark Dawe, chief executive of the OCR exam board, said: "What's very clear is changes in the science specifications have had an impact on grades, and it was known from the beginning, the Government wanted a harder science paper, when Ofqual was accrediting it, it was a harder science paper, and when the results are coming through, this is harder."

:: In English, 63.9% of entries got at least a C, compared to 65.4% last summer, while 15% were awarded an A or A*, down from 16.8% in 2011;

:: In English literature, 76.3% of exams were awarded A*-C, compared to 78.4% last year, and 23.2% got at least an A, against 25% in 2011;

:: In maths, 58.4% of entries got at least a C grade, down from 58.8% in 2011, and 15.4% got A*-A, compared to 16.5% last summer;

:: In science, 60.7% got A*-C grades, down from 62.9% and 9.8% got A*-A, down from 11.6% in 2011.